1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas, and to a process for producing the same. More particularly, it relates to a catalyst for purifying an exhaust gas, catalyst which is of good durability, and to a process for producing the same.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC, and which reduce NO.sub.x, thereby purifying the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a thermal resistant support formed of cordierite, a coating layer formed of .gamma.-alumina and disposed on the support, and a noble metal element selected from the group consisting of platinum (Pt), palladium (Pd) and rhodium (Rh) loaded on the carrier layer.
From the viewpoint of the global environment protection, carbon dioxide (CO.sub.2), which is emitted from internal combustion engines of automobiles, or the like, is at issue. In order to reduce the carbon dioxide, so-called lean-burn engines are regarded promising. In lean-burn engines, the air-fuel mixture is lean-burned in an oxygen-rich atmosphere. The fuel consumption can be reduced because lean-burn engines consume the fuel less. Accordingly, the carbon dioxide, which is emitted from lean-burn engines as one of the burned exhaust gases, is inhibited from generating.
The conventional 3-way catalysts oxidize CO and HC, and simultaneously reduce NO.sub.x to purify them. The CO, HC and NO.sub.x are produced by burning an air-fuel mixture whose air-fuel ratio is controlled at the theoretical air-fuel ration (i.e., the stoichiometric point). Consequently, the conventional 3-way catalysts do not have enough activity to remove NO.sub.x, which results from the exhaust gases produced by burning the fuel-lean-air-fuel mixture, by reduction in an oxygen-rich atmosphere (or in a fuel-lean atmosphere). Thus, it has been desired to successfully develop an automotive exhaust catalyst and a purifying system which can effectively purify NO.sub.x in an oxygen-rich atmosphere (or in a fuel-lean atmosphere).
Under the circumstances, the applicant of the present invention proposed novel exhaust catalysts, and filed the following patent applications therefor with the Japanese Patent Office. For example, Japanese Unexamined Patent Publication (KODAI) No. 5-317,652 discloses a catalyst for purifying an exhaust gas in which an alkaline-earth metal, and Pt are loaded on a porous support including alumina, or the like; Japanese Unexamined Patent Publication (KOKAI) No. 5-168,860 discloses a catalyst for purifying an exhaust gas in which lanthanum, and Pt are loaded on a porous support; and Japanese Unexamined Patent Publication (KOKAI) No. 6-31,139 discloses a catalyst for purifying an exhaust gas in which an alkali metal, and Pt are loaded on an alumina support. In these catalysts, the NO.sub.x is stored in oxides of the alkaline-earth metal, lanthanum and alkali metal (i.e., NO.sub.x storage elements) in a fuel-lean atmosphere (or in an oxygen-rich atmosphere), and the stored NO.sub.x is released at the stoichiometric point or in a fuel-rich atmosphere (or in an oxygen-lean atmosphere). The released NO.sub.x reacts with the reducing components, such as HC and CO. Thus, the catalysts can exhibit favorable NO.sub.x purifying performance even in a fuel-lean atmosphere (or in an oxygen-rich atmosphere).
When producing these catalysts, a so-called adsorption loading process is employed predominantly: namely; a solution including a compound of a catalytic noble metal is first impregnated into a porous support, such as alumina, and the porous support is dried and calcined to load the catalytic noble metal thereon. Then, a solution including a compound of an NO.sub.x storage element is impregnated into the porous support with the catalytic noble metal loaded thereon, and the porous support is dried and calcined to load the NO.sub.x storage element thereon.
Whilst, the average temperature of exhaust gases at the inlet of catalysts for purifying exhaust gases, and the maximum temperature thereof have tended to increase much higher, because the exhaust-gas emission control has been tightened, and because automobile engines have been required to exhibit high performance. Hence, in catalysts, a further heat-resistance improvement has been longed for. In addition, as the temperature of exhaust gases at the inlet of catalysts increases, a catalyst having an improved NO.sub.x purifying capability at elevated temperatures has been longed for.
However, the conventional catalysts for purifying exhaust gases suffer from a problem in that the NO.sub.x storage capability of the NO.sub.x storage elements degrades because the NO.sub.x storage elements react with the support at elevated temperatures. Moreover, it is difficult for the conventional catalysts to securely exhibit the NO.sub.x purifying capability at elevated temperatures, because they can exhibit the maximum purifying capability only in a narrow temperature range (i.e., temperature window).
In addition, in the conventional catalysts for purifying exhaust gases, the NO.sub.x storage elements are poisoned by SO.sub.x which results from the sulfur included in fuels in a trace amount, and are deteriorated in terms of the NO.sub.x storage capability by the generation of sulfates. As a result, the durability of the conventional catalysts has deteriorated.
In a process for producing the conventional catalysts for purifying exhaust gases, the NO.sub.x storage elements are loaded on the porous support by an adsorption loading process. However, the NO.sub.x storage elements are less likely to be dispersed by the process. Accordingly, the distribution of the NO.sub.x storage elements is heterogeneous, and the crystallization of the NO.sub.x storage elements develops around where the NO.sub.x storage elements are loaded in high concentrations. As a result, the NO.sub.x storage capability of the NO.sub.x storage elements has deteriorated. In particular, the NO.sub.x purifying capability of the resulting catalysts at elevated temperatures depend greatly on the combinations of the NO.sub.x storage elements and the porous support, and on the dispersibility of the NO.sub.x storage elements.
For instance, when the NO.sub.x storage elements are dispersed less, the crystals of the sulfates, which result from the sulfur poisoning, are likely to grow. It is furthermore difficult to eliminate the crystalline sulfates from the resulting catalysts for purifying exhaust gases. As a result, the durability of the resulting catalysts has deteriorated. Moreover, the alkali metals, one of the NO.sub.x storage elements, are likely to be dissipated or eluted out by water vapor involved in the exhaust gases, because they are loaded on the surface of the porous support by the conventional adsorption loading process. As a result, the durability of the conventional catalysts with the alkali metals loaded thereon has not been high satisfactorily.
Therefore, the inventor of the present invention developed an amorphous and homogeneous composite oxide support in which NO.sub.x storage elements are dispersed on the order of atomic size. The composite oxide support comprises a support component, and an NO.sub.x storage element. The support component includes an oxide of metal which is at least one element selected from the group consisting of group 3B elements, group 4A elements and group 4B elements in the periodic table of the elements. The NO.sub.x storage element includes at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements. The support component and the NO.sub.x storage element constitute and amorphous composite oxide.
It is possible to produce a catalyst for purifying an exhaust gas, which exhibits good NO.sub.x purifying performance, and which is inhibited from being poisoned by sulfur, by using the composite oxide support. For example, the composite oxide support can be pulverized to prepare a powder. The resulting composite-oxide-support powder, and another powder with a noble metal element loaded thereon can be mixed to complete such a catalyst.
In the conventional catalysts for purifying exhaust gases, a honeycomb-shaped support substrate has been used widely. The honeycomb-shaped support substrate is made from cordierite or metal, and is covered with a coating layer formed on the surface. The coating layer is formed of a support powder, such as alumina, and a catalytic component is loaded thereon. When producing the conventional catalysts, the support substrate is immersed into a slurry in which the support powder is dispersed in water, and is taken out of the slurry. Then, the excessive slurry is removed from the support substrate. Finally, the support substrate is dried, and calcined to complete the conventional catalysts.
However, the inventor of the present invention found out the following drawbacks: namely; when the slurry was prepared by dispersing the above-described composite oxide support powder to form the coating layer on the support substrate, the NO.sub.x elements, which have been dispersed homogeneously in the composite oxide support, eluted out into the slurry to degrade the dispersibility of the NO.sub.x elements in the composite oxide support. Accordingly, the resulting catalysts for purifying exhaust gases were deteriorated in terms of heat resistance and sulfur-poisoning resistance.
The drawback results from the fact that the NO.sub.x storage elements are likely to dissolve into water. Accordingly, when an organic solvent is employed to keep the NO.sub.x storage elements from eluting out, it is possible to overcome the drawback. However, the inventor of the present invention found out another drawback when a slurry was prepared by using an organic solvent. The density of the coating layer made from such a slurry has decreased to deteriorate the strength thereof, because the surface tension of organic solvent is as low as 1/3 of the surface tension of water.